To clarify the idea that an alteration of the transferrin receptor (TF-R) gene, localized to 3q26, may be of pathogenetic significance in hematological disorders with 3q anomaly, we studied the TF-R systems of erythroblasts from both functional and genetic aspects. The patient described here had refractory anemia with an excess of blasts (RAEB), with paracentric inversion, inv(3)(q21q26). The patient had the characteristic findings of micromegakaryocytosis and thrombocytosis, with giant platelets. There was no functional abnormality of TF-R as far as number of binding sites, affinity, molecular weight, or recycling kinetics were concerned. Furthermore, we could not recognize any rearrangement of the TF-R gene with Southern blot analysis. These data suggest that TF-R is not involved in the pathogenesis of leukemogenesis and thrombocytosis of the disease.